First of all, when dealing with IGS coordinates (Lat, Lon, ellipsoid height), one uses a gravimetric geoid and NOT a hybrid geoid. The US hybrid models (labeled ??GEOID##??) are based on a gravimetric model but also include corrections for datum differences.?ÿ
The gravimetric geoids developed at NGS are labeled USGG##. There are also ??experimental? gravimetric models labeled xGeoid##. They should NOT be used ?ÿwith NAD83 ellipsoid heights (though the USGG tool takes NAD83 coordinates and performs the computations in ITRF.
Not too long ago, I ran data through four on-line tools which provide the ellipsoid-geoid separation. Results differed at the decimeter level. Outputs are shown elsewhere in this thread. The URL for the xGeoid20 tool whose output includes results in the hybrid model and the future vertical datum is here:?ÿ
https://beta.ngs.noaa.gov/GEOID/xGEOID20/computation.shtml
Read base9geodesy??s cautions about ??working with? WGS84 data. NGA reports that the most recent versions of WGS agree with IGS ??on the order of 1 cm.?
As base9geodesy also states, who is asking for values in WGS84 or IGS? Do they know what they are asking for?
BTW, here are some tabulations regarding ITRF frames taken from a presentation by Stephen Malys of NGA and a screen capture of the NGA ??standardization document? describing WGS84 and its relationship to other TRF??s.?ÿ
?ÿ
In closing, I will address your explicit question about ??pairing? WGS84 and ITRF data with geoid models. ITRF coordinates can be transformed between realizations using the parameters published on their site. There are ??large? differences between the earlier versions and more recent.
Transforming ITRF coordinates (accounting for changes in translations, rotations and scale and their changes over time) will change ellipsoid heights. The NGS site has tools to perform these transformation including transformations from ITRF to NAD83.
Working with different realizations of WGS84 is more challenging with the best result obtained by considering them equivalent to ITRF. ?ÿ
As for how geoid models fit into the problem.?ÿOne uses a model of the ellipsoid-geoid separation in order to approximate an orthometric height. Their are different types of heights see: http://geodesyattamucc.pbworks.com/f/HeightSystemsSneeuw.pdf
I am not sure what a WGS84 height is intended to represent. I do know that using their tool yields results different at the decimeter level with results from NGS (xGeoid), Natural Resources Canada and UNAVCO.
If tasked to provide an orthometric height for a point whose coordinates were provided in ITRF94 at a specific epoch, I??d transform the coordinates to the reference epoch of the most ITRF use the xGeoid tool and give them the program output and a link to the xGeoid page.?ÿ
My fair city delayed moving off '29 for a long time because a) the DOT and County hadn't made the move, and b) because of the volume of legacy projects that were elevated in '29. A few years back moving to '88 was discussed and, I'm told, was almost decided on when NGS announced its intention to introduce a new datum "in 10 years". So the idea was shelved.?ÿ And then the new datum delayed maybe 5 years.?ÿ Nevertheless, with the new datum on the horizon nobody who hasn't already switched to '88 is going to do it now.
Correct. Many people do not know that. I think I have to deal with most of them.
The point I was trying to make is that while many old local BMs may have MSL values associated with them on an old map, report, or even stamped on them, there may not be any information on how the elevation was derived, nor the setting of the monument. Therefore, I cannot trust the methods by which someone established the elevation as it relates to NGVD29, nor can I trust that the monument is stable enough to maintain its original vertical position over such a long period of time. That is why I require field investigations of these BMs to establish NAVD88 Orthometric Heights.
Of course the first question that gets asked is, "So what's the conversion factor to NGVD29?" My answer? "If your BM is still in its original vertical position, and we trust that the value we were provided in your report is truly NGVD29, then the difference is A-B=Conversion Factor for this location. The closest NSRS BM with both NGVD29 and NAVD88 says the difference is X. As you can see, that differs from what we have by direct observation by 0.xx."
As I understand it, and maybe I don't, NGS will not be providing a relationship or conversion tool for NGVD29 forward, only from NAVD88 forward.?ÿ
Totally understood. We have recovered NGVD29 marks in the valley floors of California and tied them into NAVD88 based projects (not as constraints, but repurposed for project control and/or to check vertical stability). Using the newly determined NAVD88 value and applying VERTCON to get a modeled '29 elevation, we have seen MANY that were 5 or more feet lower than the superseded NGVD29 value on the datasheet. Some have exceeded 10' lower.?ÿ
I have also seen them on levees that were used as local control for a nearby stream gauge(s) that were more than 2' lower than where they had been reported to, and relied upon to be. Makes flood forecasting/modeling a little dicey using stream gauge data.